It is known that wobble can be corrected by double reflection from the active facet; see Meeussen et al., U.S. Pat. No. 3,897,132. But all previous embodiments of this concept have bow, which makes them unworkable. In copending application Ser. No. 876,648, a polygon system for correcting wobble by double reflection from the active facet, without bow, is disclosed. In that application, the angles of incidence at the first and second reflections from the facet are recognized as important to the effects of bow, and bow can be almost eliminated by the proper relation between these angles. Bow can be, for all practical purposes, entirely eliminated when the facets have a draft angle, which has a predetermined relation to the angles of incidence at the first and second reflection.
High speed printing with a single beam, double reflection, polygon scanner (dubbed Polycone.TM. by the assignee herein), means a high rotation rate for the polygon. For some applications the angular rotation rate is so great that the centrifugal stress causes unacceptable strain in the polygon, and also that the power required to drive the polygon is unacceptably large.
A multi-beam scanning system, wherein two or more beams simultaneously scan separate lines on the photoreceptor, reduces the angular rotation rate of the polygon for the same output data rate. When a polygon scanner, for which the incoming beam is offset in the scan plane, is used with a multiple beam, there is no differential bow or differential scan length between the individual beams. But if the incoming beam is offset in the cross-scan plane, there is differential bow and scan length between the multiple beams. The Polycone is offset in the cross-scan plane, and has differential bow and differential scan length, if multiple beams are used.
According to the present invention, the differential scan length between multiple beams can be eliminated by tilting the beams with respect to the photoreceptor surface, and with respect to each other. Then the beam with the shorter scan lengths when the beams are normal to the photoreceptor, travel a greater distance before they impinge on the photoreceptor. At the start or end of scan the scan angle is not zero, so the scan lengths for the beams with the larger travel distances are increased by the extra distance multiplied by the tangent of the scan angle. At the proper tilt angle, all scan lengths at the photoreceptor are the same. Since the angular scan rate of all the beams are always the same, over the entire scan, the scan lengths of the individual beams will always be the same, at any fraction of the total scan. Thus the information written onto the photoreceptor by the multiple beams will be in alignment for all simultaneous beams.